you could use the beforeShow function to reset the value like so
$("#datepicker").datepicker({
onSelect: function (dateText, inst) {
var date = $('#datepicker').datepicker('getDate');
date.setDate(date.getDate() + 6);
$('#datepicker').val('Week: ' + dateText + " to " + $.datepicker.formatDate('mm/dd/yy', date));
},
beforeShow: function(input, inst) {
if($(input).val().length > 0) {
var ar = $(input).val().match('[0-9]{2}/[0-9]{2}/[0-9]{4}');
$('#datepicker').datepicker('setDate',new Date(ar[0]));
}
}
}).attr('readonly', 'readonly');